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<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
</head>

<body>
    <script>
        var flag = true;
        var flag1 = false;
        console.log(flag + 1);
        console.log(flag1 + 1);
        // 如果一个变量没有赋值就是underfined；
        var variable
        console.log(variable + 'pink');
        console.log(variable + 1);
        // 空值null
        var space = null;
        console.log(space + 'null');
        console.log(space + 1);
    </script>
</body>

</html>